1. Field of the Invention
The present invention involves ink jet printheads, and in particular modular partial bars for fabricating full width printheads from smaller printhead subunits and full width staggered array printheads fabricated from these modular partial bars.
2. Description of Related Art
It is well known in the ink jet printhead industry to construct printheads from extended arrays of printhead subunits. The extended array approach is preferred over a monolithic approach, wherein a large printhead is constructed from a large unitary element, because the printhead subunit approach results in a higher yield of usable jets from the material used to construct the printheads (usually silicon wafers). This higher yield results because the subunit approach permits individual subunits to be tested prior to assembly into the full width printhead. For example, when constructing thermal ink jet printheads which include a plurality of nozzle defining channels having resistive heating elements therein, a single defective resistive element results in the entire full width printhead which includes over 2500 resistive elements being discarded in the monolithic approach whereas only the printhead subunit which contains the defective resistive heating element is discarded in the subunit approach. See, for example, U.S. Pat. No. 4,829,324 to Drake et.al. for a more detailed explanation of the advantages of the subunit approach over the monolithic approach.
When using the subunit approach, a plurality of printhead subunits can be butted against one another on one side of a substrate which also acts as a heat sink to form the full width extended array printhead. Alternatively, a staggered subunit approach can be used wherein a plurality of printhead subunits are arranged on opposite sides of a common substrate, such as a heat sink, so that spaces exist between each printhead subunit on each of the opposite sides of the common substrate with the printhead subunits on one side of the common substrate located opposite from the spaces on the other side of the common substrate. Although the printhead subunits 4 located on one side of substrate 2 are not capable of printing a continuous line of text which extends the full width of the recording medium (e.g. a sheet of paper), all of the printhead subunits 4 in the entire full width staggered array (i.e., the subunits 4 on both sides of substrate 2) are collectively capable of printing a continuous line of text across a sheet of paper. FIG. 1 illustrates a previous design of a full width staggered array printhead wherein a plurality of printhead subunits 4, each of which includes a plurality of nozzles 6 are arranged on opposite sides of a common heat sink substrate 2. Two ink supplying manifolds 8 are provided, one for the printhead subunits 4 located on each side of common substrate 2, to supply ink to each of the printhead subunits 4. Each printhead subunit 4 includes an ink fill hole in its surface which contacts the ink manifold 8. The/ink fill hole communicates an internal cavity of each ink manifold 8 with all of the nozzles 6 in each printhead 4. Printhead subunits usable in the full width staggered array printhead illustrated in FIG. 1 as well as for the present invention are disclosed, for example, in U.S. Pat. No. 4,786,357 to Campanelli et.al., the disclosure of which is herein incorporated by reference. A number of architectures for staggered array printheads are disclosed in U.S. Pat. No. 4,463,359 to Ayata et.al., the disclosure of which is herein incorporated by reference.
While previous staggered array printhead architectures may meet the basic requirements of a full width printhead, they have a number of practical problems. One problem is that the alignment (in X, Y, Z and respective thetas) of printhead subunits on one side of the common substrate to the printhead subunits on the other side of the common substrate is not straightforward to achieve. The two sided alignment problem is compounded by the need to cure the printhead subunits on both common substrate sides in place, which leads to complex assembly fixture configurations. Additionally, staggered array printheads constructed as in FIG. 1 also require two-sided assembly and packaging. This means that two-sided wire bonding and encapsulation, as well as two-sided assembly and sealing of the ink manifold is required. This requires assembly process research and development and removes the process from standard commercial practices. Yet another problem is a potentially heavy penalty in printhead subunits lost in rejected full width printheads. After the printhead subunits are bonded to the common substrate, one bad or clogged printhead subunit results in the disposal of the entire common substrate and all of the good printhead modules contained thereon.
U.S. Pat. No. 4,829,324 to Drake et.al. discloses a full width array ink jet printhead constructed from a plurality of printhead subunits each of which includes a heater plate having a plurality of resistive heating elements thereon and a channel plate having a plurality of nozzle defining channels formed in one surface thereof. A number of printhead subunit arrangements for forming one-sided full width printheads using the butting approach are disclosed.
U.S. Pat. No. 4,786,357 to Campanelli et.al. discloses a method for fabricating thermal ink jet printheads from two silicon wafers. A plurality of heater plate subunits are formed in one silicon wafer and a plurality of channel plate subunits are formed in another silicon wafer, the two silicon wafers are bonded to one another so that each heater plate subunit on the first silicon wafer corresponds to a channel plate subunit on the other silicon wafer. The bonded silicon wafers are then separated between each of the subunits to form a plurality of fully functional thermal ink jet printhead subunits.
U.S. Pat. No. 4,612,554 to Poleshuk discloses an ink jet printhead composed of two identical parts, each having a set of parallel V grooves anisotropically etched therein. The lands between the grooves each contain a heating element and its associated addressing electrode. The grooved parts permit face-to-face mating, so that they are automatically self-aligned by the intermeshing of the lands containing the heating element and electrodes of one part with the grooves of the other part. A pagewidth printhead is produced by offsetting the first two mated parts, so that subsequently added parts abut each other and yet continue to be self-aligned.
U.S. Pat. No. 4,534,814 to Volpe et.al. discloses a large scale printhead made up of rows of styli patterned onto glass substrates sandwiched between rugged support substrates. Multiple rows of styli are accomplished by stacking. Each row of styli is constructed from butted segments.
U.S. Pat. No. 4,863,560 to Hawkins discloses a method of fabricating channel plates for ink jet printheads from silicon wafers using orientation dependent etching techniques.